As is known, thanks to the marked development of silicon-based technologies, high-efficiency light-emitting diodes (LEDs) are increasingly used in the field of lighting, whether industrial or domestic lighting. For example, high-efficiency LEDs are commonly used in automotive applications (in particular for the manufacturing the rear lights of motor vehicles), in road signs, or in traffic lights.
According to the light intensity that it is desired to obtain, it is possible to connect alternately a number of LEDs in series or a number of arrays of LEDs in parallel (by the term array is meant, in this context, a certain number of LEDs connected in series to one another). Clearly, the number of LEDs and the criterion of connection adopted determine the characteristics of the driving device (hereinafter “driver”) that must be used for driving the LEDs.
In particular, with the increase in the number of LEDs connected in series, the value of the output voltage of the driver must increase, while, with the increase in the number of arrays in parallel, the value of the current that the driver must be able to furnish for supplying the LEDs must increase.
Furthermore, the intensity of current supplied to a LED determines its spectrum of emission and hence the color of the light emitted. It follows that, to prevent the spectrum of emission of a LED from varying, it is of fundamental importance that the supply current should be kept constant, and hence generally the driver used for driving the LEDs is constituted by a current-controlled DC/DC converter.
As is known, the topology of the DC/DC converter differs according to the type of application envisaged. Normally, the configurations “flyback” or “buck” are used, respectively, if an electrical insulation is required or if the driver is supplied directly by the electric power-supply mains (and hence there is no need to step up the input voltage), whereas the “boost” configuration is used when the driver is battery-supplied and it is hence necessary to step up the input voltage.
In many applications, it is required to vary the intensity of the light emitted by the LED gradually, this operation being known by the term “dimming”.
On the other hand, it is not possible to simply vary (either decrease or increase) the supply current supplied to the LED, in so far as it is not possible to accept the change of color of the emitted light (typically, constancy in the spectrum of emission is required), color which, as mentioned, depends upon the supply current.
For this reason, currently drivers for LEDs comprise a pulse-width-modulation (PWM) control for turning on and turning off LEDs at low-frequency (100-200 Hz), with a ratio between turning-on time and turning-off time (duty cycle) that is a function of the level of light intensity required.
To achieve turning-on and turning-off of LEDs, a switch is set in series between the output of the DC/DC converter and the LEDs themselves. Said switch, controlled in PWM, enables or disables the supply of the LEDs. In particular, during the ON phase of the PWM control signal, the switch closes, enabling passage of the supply current to the LEDs and hence their turning-on, while during the OFF phase of the PWM control signal the switch is open, interrupting passage of the supply current and hence causing turning-off of the LEDs. Clearly, the frequency of the PWM control signal is such that the human eye, given the stay time of the image on the retina, does not perceive turning-on and turning-off of the LEDs, since it perceives a light emitted in a constant way.
The circuit described, albeit enabling dimming of the LEDs to be obtained, presents, however, certain disadvantages linked to the presence of a switch connected to the output of the DC/DC converter in series with the load.
In fact, in the majority of applications, high-efficiency LEDs require high supply currents, in the region of various hundreds of mA (typically between 100 mA and 700 mA). Consequently, the switch set in series to the load must be a power switch; moreover, it must have low leakages in conduction in order not to limit the efficiency for driving. On the other hand, the higher the supply current required by the LEDs, the more critical the choice of the power switch, and consequently the higher the cost of the switch and as a whole the cost of construction of the driver.
The aim of embodiments of the present invention is to provide a LED-driving device that is be free from the drawbacks described above, and in particular that enables adjustment of the light intensity of the LEDs in a more economical and efficient way.